1. Technical Field of Invention
The present invention relates generally to an improved hemofilter for use in hemodialysis.
More particularly, the present invention relates to an improved hemofilter for use in continuous arterio-venous hemofiltration ("C.A.V.H.") having a substantially hexagonal configuration which, among other benefits, minimizes the pressure drop of the blood passing through the hollow fiber bundles of the filter to thereby reduce the likelihood of clotting within the hollow fibers of the hemofilter and, in turn, reduce the patient's need for the drug heparin during dialysis treatment.
2. Description of the Prior Art
Heretofore, hemofilters known to the prior art have generally been cylindrical in shape and having of bundles of hollow fibers for filtration. More specifically, the hemofilters currently used in C.A.V.H. were first designed for use on a dialysis machine in combination with a blood pump. Such filters, being cylindrical in geometry, have lengths in the range of approximately 10-22 cm, with relatively narrow widths in the range of 3.8-4.5 cm. Hemofilters of the prior art contain approximately 5,000 to 10,000 hollow fibers in a bundle.
The cylindrical geometry of the prior art has, however, been found to be ill-suited for a pumpless system such as C.A.V.H. The reasons for this include the fact that a long and narrow blood path through the narrow fibers leads to a drop in the perfusion pressure, which has been found to be directly proportional to the length of the fibers and inversely proportional to the fourth power of diameter of the blood path. A pressure drop in the filter leads to a drop in the blood flow rate, stagnation of red blood cells and platlets and leads to clot formation. This can be partly overcome by using higher does of heparin. However, high does of heparin can lead to bleeding on the part of the patient.
It is further known in nephrology that a cooling of fluid through a hemofilter leads to an increase in fluid viscosity, which in turn leads to an increase in flow drag and loss of kinetic energy by conversion to heat. See, Keller, K. H., Fluid Mechanics and Mass Transfer in Artificial Organs, Special Publication by Trasc. Amer. Soc. Art. Int. Organs, Georgetown University Press April 1973).
The foregoing considerations are taken into account and described by the so-called Reynold's Number ("Re") ##EQU1## wherein, p=pressure within the tube;
u=flow volume through the tube; PA1 D=inner diameter of the tube; and, PA1 .mu.=viscosity of the fluid within the tube.
The Reynold's Number should be maintained as high as practical in order to attain maximum efficiency of the hemofilter system.
While it should be theoretically possible to further increase the efficiency of C.A.V.H. systems by use of a streamline hemofilter that is shorter in length and having an increased number of hollow fibers, and thereby increase the Reynold's Number of the hemofilter system, in practice, it has been found that blood entering the hemofilter passes more quickly and efficiently through the hollow fibers closest to the inlet port and less efficiently through those hollow fibers which are a greater distance from the blood ports of the device. In other words, the greater the distance that a particular hollow fiber is from the blood ports of the hemofilter, the less efficient and effective is the filtering process of the system with respect to such fibers; blood having a tendency of seeking the more centrally located fibers of the system.
There is a tendency of the blood to less readily pass through fibers which are located away from the blood ports of the hemofilter, which results in clot formation and thereby obstructs the flow of blood through those fibers and reduces the efficiency of the filter. As may be noted from the Reynold's Number formula, an increase in the inner diameter of the tube ("D"), i.e., total number of hollow fiber parts available for blood flow, would beneficially increase the Reynold's number and increase the efficiency of the hemofilter system. The current hemofilters of cylindrical design, when used in a horizontal position, have red cell and platelet sedimentation in their dependent parts due to gravity, and this places severe limitations upon the extent to which presently known hemofilters may have the inner diameters, and thereby their efficiencies, increased.